(a) Field of the Invention
The present invention relates to a method for selectively and sequentially extracting catechins from a plant product, and more particularly from green tea leaves.
(b) Description of Prior Art
Tea is the second most consumed beverage around the world, after water, for its attractive aroma and flavor, as well as for its health-related benefits. The health-related benefits attributed to tea are antioxidant properties, stimulation of detoxification (phase II) enzyme activity, inhibition of transcription factor activation, inhibition of cell-signaling pathways, induction of apoptosis, cell cycle regulation, inhibition of cell invasion and angiogenesis, and interference with receptor binding.
For many years, numerous works were carried out to characterize the composition of tea infusions responsible for those health-related benefits. A typical green tea infusion, i.e. 1 gram of tea leaves in 100 mL of water for a three (3) minutes brew, comprises approximately 250 mg of solids, wherein approximately 30% are catechins, while a black tea infusion catchechin content is approximately 9%.
Catechins are colourless, water-soluble compounds which impart bitterness and astringency to tea infusion. These molecules belong to flavonoids, which are plant secondary metabolites distributed in the plant kingdom. Flavonoids can be subdivided into six classes: flavones, flavanones, isoflavones, flavonols, flavanols and anthocyanins based on the structure and conformation of the heterocyclic oxygen ring (C ring) of the basic molecule. Tea catechins are primarily flavanols and more principally epicatechin (EC), epigallocatechin (EGC), epicatechin gallate (ECG), and epigallocatechin gallate (EGCG). EGCG is regarded as the most important of the tea catechins because of its high content in tea and the fact that its activity is mirrored by green tea extracts.
Methods for producing tea extracts with high EGCG ratios have been reported in the prior art (Copland et al., 1998. Food Chem. 61:81-87). Du et al. (Du et al., 1997. res. Develop. Basic Agric. and High Technol., 1:40-47; Du et al., 1998. J. Liq. Chromatog. & Related Technol., 21:203-208) reported the use of high-speed counter-current chromatography (HSCCC) as fractionation tool for both crude extracts and semi-purified fractions as well as for the production of purified catechin from crude green tea extracts.
Patent application CN 141426, Applicant HUBEI Provincial Institute of Chemistry, reports a process for preparing tea polyphenol with high catechin content and low caffeine content that comprises an alcohol precipitation step and an alcohol co-precipitation step.
Patent application CN 1370766, Applicant Zhongnan University, reports a pollution-free polyphenol extraction process from tea that comprises salting out a tea infusion using a polyamide as adsorbent and separating polyphenols from caffeine and pigment using an acid aqueous solution of citric acid as washing agent.
Methods for the extraction of flavonoids or catechins from sources other than tea leaves are also known in the art. For example, patent application CN 1241447, Applicant Zhejiang University, reports a flavonoid extraction process from scutellaria root. This process comprises leaching out flavonoids from scutellaria roots with a water solution containing an ethylene oxide-propane oxide random copolymer (EOPO) and ethanol. This step is followed by subsequent concentration and purification steps, including a temperature changing counter-extraction process.
Patent application CN 1228968 reports a method for extracting flavonoid compounds from bamboo leaves that comprises a thermal reflux extraction of bamboo leaves with low-concentration low-grade alcohol. This step is completed by a concentration step, a separation and refining step by liquid-liquid extraction using middle-grade alcohol and finally, drying.
Although the above-mentioned methods may lead to the obtention of high yield catechins and other flavonoids from plant extracts, they required the use of organic solvent or adsorbent or are restricted to the purification of limited quantities of catechins, especially when a liquid chromatography (HSCCC) is required. Therefore, these methods are not easily transposable to the industrial scale since they are not environmentally nor economically advantageous. Moreover, in an industrial context of producing a catechin extract or an EGCG-enriched fraction, an important step is the solubilization of catechins from tea leaves.
Kinetics and equilibria of black tea infusion have been widely studied by Spiro et al. until recently. However, it is widely established in the prior art that the polypheriol content of black tea significantly differs from that of green tea. Fresh tea leaves are rich in catechins. Tea leaves also contain polyphenol oxidase enzymes in separate compartments from catechins. When tea leaves are rolled or broken up during industrial manufacture, catechins contact polyphenol oxidase, joining them to one another and forming theaflavins and thearubigins. These oxidized tea polyphenols give oolong and black tea their distinct flavors and colors. Steaming or pan firing inactivates the polyphenol oxidase enzyme. During its manufacture, white and green tea is steamed to inactivate polyphenol oxidase and is then dried. Thus, white and green tea contains higher concentrations of catechins. In contrast, black tea is highly oxidized, resulting in increased concentrations of theaflavins and thearubigins and relatively low catechin concentrations, while oolong tea is only partially oxidized. The flavonol content is less affected by processing, and flavonols are present in comparable quantities in white, green, oolong, and black teas. On the aspect of solubilization kintetics and equilibra of catechin, very little basic scientific data are available on the technology of brewing of green tea, which may significantly differ from black tea.
Since green tea represent a much proper raw material for obtaining large amounts of catechins, it would be highly desirable to be provided with a environmentally-friendly and economical method, that is transposable to an industrial scale for producing catechin enriched fractions from green tea leaves.